The present invention relates to a frequency measuring device capable of measuring a frequency highly accurately and continuously within a short period and to a device and method for polishing by using the frequency measuring device.
Further, the present invention relates generally to an eddy current sensor, and more particularly, to an eddy current sensor which is capable of detecting an eddy current loss produced in a conductive film made of copper (Cu) or the like deposited on the surface of a substrate such as a semiconductor wafer, when the conductive film is polished by a chemical mechanical polishing (CMP) technique, to detect the advancement of polishing, and a method of detecting a polished thickness using the eddy current sensor.
A chemical mechanical polishing (CMP) process has been widely employed as a method in which a semiconductor substrate is dipped in a plating solution to conduct, for example, electrolytic plating or non-electrolytic plating to form an electrically conductive film, and thereafter an unnecessary electrically conductive film on the surface of the semiconductor substrate is removed. Hereinafter, with reference to FIGS. 1 to 3, a description will be given of the structure and operation of a polishing device for the CMP process proposed by the applicant of the subject application. FIG. 1 is a longitudinal cross-sectional view showing the entire structure of the polishing device. The polishing device is equipped with a turn table 101, and a top ring 102 that presses a semiconductor wafer 103 against the turn table 101 while retaining the same. The turn table 101 is coupled to a motor 104 so as to be rotatable about a shaft center of the motor 104 as indicated by an arrow. A polishing cloth 105 is attached to the top surface of the turn table 101.
The top ring 102 is coupled to a motor and an elevating cylinder (not shown) through a top ring shaft 106. With this structure, the top ring 102 can be elevated along the top ring shaft 106 in a direction indicated by an arrow and is rotatable about the top ring shaft 106 so that the semiconductor wafer 103 can be pressed against the polishing cloth 105 with an arbitrary pressure. An elastic mat 107 made of polyurethane or the like is disposed on the lower surface of the top ring 102. A guide ring 108 for latching the semiconductor wafer 103 is disposed on the outer peripheral portion of the lower portion of the top ring 102.
A polishing abrasive solution nozzle 109 is located above the turn table 101, and a polishing abrasive solution Q is supplied to the polishing cloth 105 stuck onto the turn table 101 by the polishing abrasive solution nozzle 109.
As shown in FIG. 1, an eddy current sensor 110 that constitutes an end point detecting mechanism for detecting an end point in the polishing of the semiconductor wafer 103 is embedded within the turn table 101. A wiring 111 of the eddy current sensor 110 passes through the turn table 101 and a turn table support shaft 112 and is then connected to a controller 114 through a rotary connector (or slip ring) 113 disposed at a shaft end of the turn table support shaft 112. The controller 114 is connected to a display device (display) 115. Note that, instead of embedding the eddy current sensor 110 within the turn table 101, the eddy current sensor 110 may be embedded within the top ring 102. In the case where the eddy current sensor 110 is embedded within the top ring 102, the connection between the eddy current sensor 110 and the controller 114 is effected through a slip ring (not shown) disposed at an appropriate position of the top ring shaft 106.
FIG. 2 is a plan view showing the positional relationship of the turn table 101, the semiconductor wafer 103 and the eddy current sensor 110 in the polishing device shown in FIG. 1. As shown in the figure, the eddy current sensor 110 is located at a position that passes through a center Cw of the semiconductor wafer 103 that is held by the top ring 102 while being polished. Reference symbol Ct indicates the center of rotation of the turn table 101. The eddy current sensor 110 sequentially detects the thickness of the electrically conductive film such as a Cu layer of the semiconductor wafer 103 on a passing locus during a period where the eddy current sensor 110 passes below the semiconductor wafer 103.
FIGS. 3a and 3b are enlarged main-portion cross-sectional views showing a state where the eddy current sensor 110 is embedded. FIG. 3a shows a case where the polishing cloth 105 is attached to the turn table 101, and FIG. 3b shows a case where a fixed abrasive grain plate 105′ is located on the turn table 101. In the case where the polishing cloth 105 is attached to the turn table 101 as shown in FIG. 3a, the eddy current sensor 110 is embedded within the turn table 101. On the other hand, in the case where the fixed abrasive grain plate 105′ is located on the turn table 101 as shown in FIG. 3b, the eddy current sensor 1 is embedded within the fixed abrasive grain plate 105′.
In either one of FIGS. 3a and 3b, a distance L from the polishing surface which is the top surface of the polishing cloth 105 or the polishing surface which is the top surface of the fixed abrasive grain plate 105′, namely, the surface (lower surface) of the semiconductor wafer 103 to be polished, to the top surface of the eddy current sensor 110 may be set to 1.3 mm or more. In FIGS. 3a and 3b, there is shown the semiconductor wafer 103 where an electrically conductive film 103b consisting of a Cu layer or an Al layer is formed on an oxide film (SiO2) 103a. 
The polishing cloth 105 is made of, for example, non-woven fabric such as Politex manufactured by Rodale Corp. or foam polyurethane such as IC 1000. Also, the fixed abrasive grain plate 105′ is formed by solidifying fine abrasive grains of several μm or less in the degree of grains (for example, CeO2) using resin as a bonding agent and forming them into a disk shape.
In the polishing device shown in FIGS. 1, 2 and 3a, the semiconductor wafer 103 is held on the lower surface of the top ring 102, and the semiconductor wafer 103 is pressed against the polishing cloth 105, or against the fixed abrasive grain pack 105′ of FIG. 3b, on the top surface of the rotating turn table 101 by the elevating cylinder. By supplying the polishing abrasive solution Q from the polishing abrasive solution nozzle 109, the polishing abrasive solution Q is retained on the polishing cloth 105, and polishing is conducted in a state where the polishing abrasive solution Q exists between the surface (lower surface) of the semiconductor wafer 103 to be polished and the polishing cloth 105. The eddy current sensor 110 passes immediately below the surface of the semiconductor wafer 103 to be polished each time the turn table 101 makes one rotation. In this case, because the eddy current sensor 110 is located on the locus that passes through the center Cw of the semiconductor wafer 103, the eddy current sensor 110 can continuously detect the thickness of the electrically conductive film on the arcuate locus of the surface of the semiconductor wafer 103 in accordance with the movement of the eddy current sensor 110. Note that, in order to reduce the detection interval, indicated by the imaginary line of FIG. 2, other eddy current sensors 110′ may be added so that two or more eddy current sensors 110′ are disposed on the turn table 101.
A brief description will be given here of the principle on which the thickness of the electrically conductive film on the semiconductor wafer 103 formed of a Cu layer or an AL layer is detected by using the eddy current sensor 110 to judge the end point of the CMP process. When a high frequency current flows through the sensor coil of the eddy current sensor 110 to generate an eddy current in the electrically conductive film (metal film) of the semiconductor wafer 103, the eddy current sensor 110 and the semiconductor wafer 103 are magnetically coupled to each other by a mutual inductance. In this state, since the synthetic impedance of the sensor circuit of the eddy current sensor 110 and the conductive film of the semiconductor wafer 103 becomes a function of the resonance frequency of an oscillating circuit within the eddy current sensor 110, the resonance frequency is monitored to thereby make it possible to detect the end point in polishing.
Assume an equivalent circuit of the eddy current sensor 110 and the semiconductor wafer 103, wherein the inductance of the eddy current sensor 110 is L1, the resistance of the eddy current sensor 110 is R1, the inductance of the semiconductor wafer 103 is L2, the resistor of the eddy current sensor 110 is R2, and the mutual inductance that magnetically couples both of the eddy current sensor 110 and the semiconductor wafer 103 is M. Also, assuming that j is the square root of −1 (imaginary number), f is the resonance frequency of the eddy current sensor 110, and Ω=2πf, then the above synthetic impedance Z is represented as follows:Z=(R1+Rx·R2)+j Ω)(L1−Rx·L2)Rx=Ω2M2/(R22+Ω2L22)Therefore, the synthetic impedance Z changes with a change in Rx and the resonance frequency f of the eddy current sensor 110 also changes simultaneously. By monitoring the degree of the change in the resonance frequency f, it is possible to judge the end point of the CMP process.
A detection signal outputted from the eddy current sensor 110 is processed by the controller 114 while the semiconductor wafer 103 is polished by the polishing device shown in FIG. 1 on the basis of the above-mentioned principle, and a change in the resonance frequency f obtained as a result of the above processing is observed. FIG. 4 shows an example of a graph in which an abscissa axis represents a polishing time (sec.), and an ordinate axis represents the resonance frequency f (KHz) of the eddy current sensor 110, respectively. Therefore, the graph of FIG. 4 shows a change in the resonance frequency f when the eddy current sensor 110 passes immediately below the semiconductor wafer 103 a plurality of times. Note that the electrically conductive film on the semiconductor wafer 103 when the above graph is obtained is formed of a Cu layer.
As shown in FIG. 4, as polishing is advanced, that is, as the thickness of the conductive film is reduced, the resonance frequency f obtained as a result of processing the detection signal of the eddy current sensor 110 by the controller 114 continues to reduce (FIG. 4 shows that the resonance frequency f is gradually reduced from 6800 Hz). Therefore, by obtaining in advance the resonance frequency when the conductive film other than the wiring conductors is removed from the semiconductor wafer 103, the resonance frequency f of the eddy current sensor 110 is graphed as shown in FIG. 4, and the end point of the CMP process can be detected from a change in the inclination of the resonance frequency at the respective polishing times. In the graph shown in FIG. 4, the resonance frequency when the unnecessary conductive film is removed from the upper surface of the semiconductor wafer 103 is 6620 Hz.
Also, it is possible to set a certain frequency before the frequency reaches the end point of polishing that is set as a threshold value, and to perform polishing by using the fixed abrasive grain plate 105′ (refer to FIG. 3b) with a high polishing speed (polishing rate) until the frequency reaches the threshold value. Polishing is performed by using the polishing cloth 105 (refer to FIG. 3a) with a lower polishing rate than that of the fixed abrasive grain plate 105′ after the frequency reaches the threshold value, and the CMP process can be completed when the frequency reaches the frequency at the time the unnecessary conductive film is removed from the upper surface of the semiconductor wafer 103.
In this way, in the CMP process, when the unnecessary electrically conductive film on the semiconductor wafer is removed, if the conductive film is over-polished, a conductor for forming a wiring circuit is also removed, which results in a semiconductor wafer being produced that cannot be used. Also, since an insufficient polishing of the electrically conductive film cannot remove the conductive films that short-circuit between the wiring circuits, it is necessary to further continue polishing, thereby resulting in an increase in the manufacturing cost. Under the above circumstances, there is a required means for detecting the thickness of the electrically conductive film formed on the polished surface of the semiconductor wafer continuously and highly precisely at a real time to accurately determine the end point of polishing.
In order to detect such a change in the resonance frequency, the output of the eddy current sensor is usually applied to a frequency measuring device. FIG. 5a schematically shows the structure of a frequency measuring device FC of a direct counting system conventionally used as an example of the above frequency measuring device, and FIG. 5b shows a signal waveform in FIG. 5a. 
In FIG. 5a, a measured signal Vin having a certain frequency is supplied to a signal input terminal IN. The measured signal Vin is a pulse wave obtained by TTL-level converting a sine waveform signal, and has such a shape, for example, as shown by Vin of FIG. 5b. After the measured signal Vin has been amplified by an amplifier A, the measured signal Vin is supplied to one input terminal of a gate circuit G. A time reference signal T outputted from a time reference circuit TR is supplied to the other input terminal of the gate circuit G. As shown in FIG. 5b, the duration of the time reference signal T is t, and the time reference circuit TR supplies the time reference signal T to the gate circuit G at given intervals. As a result, the gate circuit G is opened only during a period of the duration when the time reference signal T is supplied, and the measured signal Vin that has passed through the gate circuit G in a period at which the gate circuit G is opened is applied to a decimal counter DC so as to measure the frequency of the measured signal Vin. The measured result is latched by a latch circuit LC and then supplied to a given processing circuit such as a CPU.
Assuming that the number of pulses of the measured signal Vin that has passed through the gate circuit G in a period of the duration t when the gate circuit G is open is c, the frequency f (Hz) of the measured signal Vin at this time is represented as follows:f=c/t
In the case where the frequency measurement having five significant digits is intended to be conducted in a conventional frequency measuring device shown in FIG. 5a, the duration t of the time reference circuit TR can be set to the order of 100 ms, and the time reference circuit TR outputs the time reference signal T whose duration t is, for example, 390 ms at intervals of 390 ms. This means that the measured results of the frequency can be obtained every 390 ms, and therefore the conventional eddy current sensor can detect a change in the thickness of the electrically conductive film on the semiconductor wafer at the intervals of 390 ms.
However, the actual speed of the CMP process is increasing, and in order to accurately detect the end point in polishing and improve the yield, there is a demand for a device and method in which the result of the frequency measurement is supplied at intervals of less than 390 ms to make it possible to more accurately detect the end point in polishing in the present circumstances.
As described above, in order to form a wiring circuit on a semiconductor substrate, a method has been proposed in which grooves for wires of a predetermined pattern are previously formed and the substrate is immersed in a plating solution, for example, for electroless plating or electrolytic plating of copper (Cu), and subsequently unnecessary portions of the plated copper is removed from the surface by a chemical mechanical polishing (CMP) process. Such deposition of a copper layer by plating enables the wiring grooves of a high aspect ratio to be uniformly filled with a highly conductive metal. The CMP process involves pressing a semiconductor wafer held by a top ring onto an abrasive cloth adhered on a turn table, and simultaneously supplying a polishing solution containing abrasive grain to polish a Cu layer on the semiconductor wafer.
When the Cu layer is polished by the CMP process, the Cu layer must be selectively removed from the semiconductor substrate while leaving only portions of the Cu layer which are formed in the grooves for wiring. Specifically, except for the grooves for wiring, the Cu layer must be removed until an oxide film (SiO2) is exposed. In this event, if the Cu layer is excessively polished to remove the Cu layer in the grooves for wiring together with the oxide film (SiO2), a resulting increase in circuit resistance would lead to the abandonment of the entire semiconductor substrate, thereby sustaining a great damage. On the other hand, the Cu layer remaining on the oxide film due to insufficient polishing would fail to fully separate wiring circuits, thereby resulting in short-circuiting. Consequently, the polishing process must be performed again, thereby causing an increase in manufacturing cost. This situation is not limited to the Cu layer but is likewise possible with any other conductive film such as an Al film which is formed and polished by the CMP process.
To address the foregoing problem, a method of detecting a polishing end point using an eddy current sensor has been proposed for detecting an end point for a CMP process. FIG. 6 illustrates a main portion of a conventional polishing apparatus which comprises an eddy current sensor. The illustrated polishing apparatus comprises a rotatable turn table 121 having adhered thereon an abrasive cloth 122 which has a polishing surface on the top of the abrasive cloth 122; a top ring 123 for rotatably holding a semiconductor wafer 124, which is a substrate subjected to polishing, to press the semiconductor wafer 124 onto the abrasive cloth 122; and a polishing solution supply nozzle 125 for supplying the abrasive cloth 122 with a polishing solution Q. In place of the abrasive cloth 122, an abrasive plate formed of a resin having abrasive grains, called “fixed abrasive grains,” may be adhered on the turn table 121. The top ring 123 is coupled to a top ring shaft 126 and comprises an elastic mat 127 made of polyurethane or the like applied on its bottom such that the semiconductor wafer 124 is held in contact with the elastic mat 127. The top ring 123 further comprises a cylindrical retainer ring 128 around its outer periphery for preventing the semiconductor wafer 124 from coming off the bottom of the top ring 123 during polishing. The retainer ring 128 is fixed to the top ring 123, with its lower surface formed protrusively from a holding surface of the top ring 123, such that the semiconductor wafer 124 is held in the holding surface and prevented from coming out of the top ring 123 by an abrasive force with the abrasive cloth 122 during the polishing.
The top ring 123 also comprises an eddy current sensor coil 129 embedded therein which is connected to an active element unit 130, which comprises an oscillator circuit, through a wire 131 which extends through the top ring shaft 126, and is further connected to a processor 132 through an interface board 133 including a filter circuit, and a distribution box 134 including a waveform converter circuit. In the distribution box 134, an oscillating signal is converted to a TTL level (0–5 volts), and the oscillating frequency is counted by a frequency counter within the processor 132. The oscillating frequency thus counted is displayed on a display unit 135. The eddy current sensor coil 129 and the active element unit forms an eddy current sensor.
The semiconductor wafer 124 is held below the elastic mat 127 on the bottom of the top ring 123, and the semiconductor wafer 124 is pressed onto the abrasive cloth 122 on the turn table 121 by the top ring 123, while the turn table 121 and the top ring 123 are rotated to polish the semiconductor wafer 124 with the abrasive cloth 122 through relative movements therebetween. In this event, the polishing solution Q is supplied onto the abrasive cloth 122 from the abrasive solution supply nozzle 125. The abrasive solution, suitable for polishing Cu (copper), for example, may be an oxidizer suspended with abrasive grains comprising fine particles such as alumina or silica. The semiconductor wafer is polished by a combination of the action of the polishing solution which oxidizes the surface of the Cu layer through a chemical reaction and the action of the mechanical polishing action provided by the abrasive grains.
During the polishing, the eddy current sensor 129, 130 continues to detect a change in the thickness of a conductive film such as the Cu layer formed on the polished surface of the semiconductor wafer 124. Then, a signal from the eddy current sensor 129, 130 is monitored to detect an end point for the CMP process, relying on a change in frequency which occurs when the conductive film on an oxide film (SiO2) has been removed while leaving only the conductive material such as the Cu layer formed in grooves for wiring.
As mentioned above, the eddy current sensor is comprised of the sensor coil 129 which is positioned opposite to a substrate subjected to polishing, and the oscillator circuit (active element unit) 130 connected to the sensor coil 129 and including a capacitance and an active element. As the active element unit 130 is supplied with DC power, the sensor coil 129 and capacitance form a tank circuit which oscillates at its oscillating frequency with the active element such as a transistor. Magnetic flux formed by the sensor coil 129 extends through the conductive film on the substrate 124 placed in front of the sensor coil 129 and alternately changes to generate an eddy current in the conductive film. Then, this eddy current flows through the conductive film to produce an eddy current loss, causing a reduction in a reactance component of the impedance of the sensor coil 129 from a view point of equivalent circuit.
Thus, when the eddy current loss is zero, the oscillator circuit oscillates at the oscillating frequency of the tank circuit. However, as the eddy current loss exists, a resistance component in the oscillator circuit is increased due to the influence of an equivalent resistance component of a semiconductor wafer, thereby causing the oscillating frequency to move toward a higher region. Therefore, by observing a change in the oscillating frequency of the oscillator circuit, it can be seen that as a conductive film is gradually removed with the advancement of the polishing, the oscillating frequency is correspondingly reduced, and at the time the conductive film is completely removed by the polishing, the oscillating frequency becomes equal to the self oscillating frequency of the tank circuit, followed by a substantially uniform oscillating frequency. In this way, it is possible to detect an end point in the chemical mechanical polishing of a conductive film by detecting the point at which the oscillating frequency becomes equal to the self oscillating frequency of the tank circuit.
According to the end point detection for the chemical mechanical polishing process utilizing the eddy current sensor, the advancement of polishing performed on a conductive film can be known during the polishing without contacting a substrate subjected to the polishing.
The foregoing apparatus has the sensor coil 129 disposed in the top ring 123 for holding a substrate subjected to polishing, and the active element unit 130 of the oscillator circuit disposed in a fixture for holding the top ring shaft 126 spaced away from the sensor coil 129, wherein the two components are connected through the communication line 131. An oscillating signal formed by the oscillator circuit is introduced into a personal computer through the interface box 133, distribution box 134 and the like by a communication line 136, so that a transition of the oscillating frequency is displayed on a monitor screen of the computer. The communication line 136 is comprised of a total of four wires which include a pair of signal lines and a pair of DC power lines. Also, the communication line 131 is connected to the sensor coil 129 which is contained in the top ring for rotating the fixed active element unit 130 using a rotary connector. Thus, as the polishing of a conductive film advances, an eddy current loss decreases, thereby making it possible to observe how the oscillating frequency changes on the screen of the monitor 135 of the personal computer 132.
The conventional method of detecting a polishing end point based on the eddy current sensor, however, suffers from the following problems. Specifically, since the sensor coil 129 and active element unit 130 are placed at separate positions and are interconnected by the high impedance communication line 131 through the rotary connector, the communication line 131 picks up noise associated with the rotation of the turn table and the like. The removal of the noise is difficult in the course of output signal processing in the oscillator circuit. For this reason, a filter circuit or the like is required for attenuating a sufficient amount of noise. The communication line 136 also picks up noise.
Further, the oscillating frequency used for the eddy current sensor is relatively low, i.e., approximately 7 MHz, so that a large eddy current loss can be detected when a conductive film subjected to polishing has a sufficient thickness, whereas when the eddy current loss becomes smaller as the conductive film is polished more so that its thickness becomes extremely smaller, in which case difficulties are encountered in detecting, for example, a thickness of approximately 1000 Å or less. In other words, because of a relatively low oscillating frequency utilized for detection, the conventional eddy current sensor fails to provide a sufficiently high accuracy for detecting an end point for polishing performed by a polishing apparatus which requires a thickness detection accuracy on the order of angstroms.